Ionization chamber



IONIZATION CHAMBER Filed Dec. 11, 1945 il g:

' v INVENTOR.

fit; .5. J Howard Parsons BY Maw Patented Jan. 11, 1949 IONIZATION CHAMBER J Howard Parsons, Oak Ridge, Tenn, assignor'to the United States of America as represented by the United States Atomic Energy Commission Application December 11, 1945, Serial No. 634,315

This invention relates to an arrangement for reducing losses in high frequency electrical circuits and more particularly to circuit arrangements for transmitting high frequency pulses with a minimum of attenuation.

In apparatus such as parallel plate ionization chambers and concentric conductors which furnish relatively high frequency pulses to amplifying equipment the input impedance increases with the frequency which results in greater attenuation of the high frequency components than of the low frequency components.

An object of the invention is to provide an ionization chamber so constructed that it will provide a low impedance input when connected to an electronic counting circuit.

Another object of the invention is the provision of a capacity guard for an ionization chamber, a concentric cable and the like.

Another object of the invention is the provision of a circuit arrangement whereby an apparatus with a capacity guard may be connected to an amplifying circuit in a manner to minimize attenuation of high frequency pulses.

Other objects and advantages of the invention will become evident from the following description when read in connection with the accompanying drawing in which Fig. 1 is a schematic wiring diagram showing a prior art ionization chamber and measuring circuit;

Fig. 2 is an equivalent circuit representative of the circuit of Fig. 1;

Fig. 3 is a partial view in perspective and with parts torn away of an ionization chamber incorporating the invention;

Fig. 4 is a schematic wiring diagram showing the chamber of Fig. 3 connected to an electrometer;

Fig. 5 is an equivalent circuit representative of the circuit of Fig. 4; and

Fig. 6 shows in perspective a cable constructed and connected in accordance with the invention.

Referring to Fig. 1 there is shown a conventional type of ionization chamber having two parallel plates l0 and II comprising, respectively, a high voltage electrode and a collecting electrode, and surrounded by a grounded outer cylindrical shell l2. The collecting electrode II is connected to the grid I3 of a vacuum tube l4 4 Claims. (Cl. 25083.6)

having a grid resistor l5 and a cathode resistor l6. The normal input circuit just described can be resolved roughly into the equivalent circuit shown in Fig. 2 in which er is the theoretical voltage pulse available Re is the equivalent chamber resistance C1 is the capacity of the collecting electrode to ground R1 is the grid resistor l5, and

en is the signal to the grid 13.

Since R1 must be very large, Zg will depend on frequency components of the pulses and high frequency components are attenuated more than low frequency components. Such attenuation is not desired.

In accordance with the present invention as embodied in Figs. 3 and 4 a capacity guard in the form of a hollow cylinder [1 surrounding the electrodes l0 and I I and inside the grounded shell I2 is provided to reduce high frequency losses as hereafter described. This capacity guard I! when connected to the cathode of the tube I4 renders the input impedance substantially resistive and thus not frequency discriminative. This will be evident from a consideration the equivalent circuit of Fig. 4 shown in Fig. 5.

In Fig. 5 the terms employed are the same as those of Fig. 2 except that C1 is the capacity between the collecting electrode H and the guard I'I.

Ck is the capacity between the guard 11 and ground and Bi; is the cathode resistor It.

It will be seen that the capacity between the collecting electrode II and shell I2 is negligible because of the presence of guard ll. Thus with the capacity guard l I connected to the cathode of the cathode follower tube I4 and with the resistor l6 (Rk) small in value, the impedance of the cathode circuit remains nearly constant within the frequency band ordinarily encountered in ionization chambers. Additionally, 20 remains nearly constant since for small changes of Rk the gain is practically constant.

From the above it will be seen that the provision of a capacity guard connected to the oathode follower circuit considerably reduces the high frequency component losses in an ionization chamber and reduces resolution loss.

The use of a capacity guard as above described is not restricted to ionization chambers but may be employed generally in electrical apparatus where it is desired to minimize high frequency losses such as, for example, in a cable as shown in Fig. 6 where instead of the usual tubular conductor enclosing a central rod or wire, a central conductor 18 is surrounded by two spaced concentric conductors or conducting sheaths l9 and 20, there being insulation between the sheaths l9 and 20 and between conductor I8 and sheath 19 as shown. In this arrangement a cathode follower circuit is employed with the central conductor 18 connected to the grid of a tube 2| and the intermediate sheath or conductor I 9 connected to the cathode of the tube 21, the outer tubular sheath or conductor 20 being grounded.

While for the purpose of explaining the invention, two particular embodiments have been described, it is to be understood that many modifications will be apparent to those skilled in the art and all such modifications are intended to be included in the scope of the appended claims.

I claim:

1. An ionization chamber comprising a pair of parallel plate electrodes spaced in confronting relationship, a hollow cylindrical electrically conductive member surrounding the interelectrode space of said electodes, a second hollow cylindrical electrically conductive member surrounding said first-mentioned hollow cylindrical member and being grounded, and an electronic amplifying circuit having a cathode follower circuit,

the cathode being connected to said first-mentioned hollow cylindrical member.

2. An ionization chamber comprising an outer shell, a pair of plate electrodes spaced in parallel relation within the container, and a ring guard of conducting material insulated from and positioned between the outer shell and said electrodes and forming a shield between said electrodes and outer shell.

3. In an ionization chamber having spaced electrode members one of which is a collecting electrode, a casing of electrically conductive material around said electrodes, the casing being grounded, and an electronic amplifying cicuit having its input coupled to said collecting electrode, the improvement which comprises an additional electrical shielding member interposed between said collecting electrode and said casing around said collecting electrode and said additional electrical member having a connection to the cathode of said amplifying circuit.

4. The structure of claim 3 wherein the amplifying circuit is of the cathode follower type.

J HOWARD PARSONS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,961,717 Thomas June 5, 1934 2,033,274 Burton Mar. 10, 1936' 2,105,305 White Jan. 11, 1938 2,178,985 Blumlein Nov. 7, 1939 2,206,638 Koch July 2, 1940 2,282,319 Brown May 12, 1942 2,288,718 Kallmann et al July 7, 1942 2,302,247 Neufeld Nov. 17, 1942 2,368,486 Mullane Jan. 30, 1945 2,379,168 McClellan June 26, 1945 2,397,074 Hare et al. Mar. 19, 1946 

